1. Field of the Invention
This invention relates to thermionic emitters and more particularly to positive ion emitters for use in instruments such as ion mobility spectrometers.
2. Description of the Prior Art
U.S Pat. No. 2,742,585 which issued on Apr. 17, 1956 to P. D. Zemany describes an electrical vapor detector. A thin refractory coating a few mills thick of specific metal oxides act both as insulators and alkali ion emitters at temperatures ranging from about 700.degree. C. to 1200.degree.-1300.degree. C. or higher. The refractory coating may be oxides of aluminum (alumina), titanium (titania), beryllium (beryllia), thorium (thoria), magnesium (magnesia), calcium, molybdenum, iron, manganese, silicon, cobalt, nickle and the rare earths (the rare earths have atomic numbers 57 to 71, inclusive). In operation, at a temperature above 700.degree. C. the initial ion current from the refractory coating subsides; the device is then prepared to detect vapors of halogens and their compounds in a vacuum system of 1 mm Hg. The admission of the vapors of halogens and their compounds to the surface of the refractory coating causes an increase in the positive ion current collected upon the negatively charged collector. The electrical vapor detector detects halogens and their compounds due to an increase in evaporation of alkali ions from the surface of the coating.
In U.S. Pat. No. 2,806,991 which issued on Sept. 17, 1957 to W. P. White, an electrical detector is described for the detection of certain substances or impurities in gases. The detector comprises a double helical wire heater winding wound on a cylindral ceramic core which has been impregnated with a solution of sodium hydroxide. An electrode inserted into tight fitting holes in the ceramic core which act as the cold electrode. The ceramic core must be impregnated with a highly conductive salt such as NaOH, NaF, or LiCl. The vapor dectector is particularly adapted to detect the presence of hydrogen, in flammable gases, reducing gases, or vapors containing hydrogen.
In U.S. Pat. No. 3,972,480 which issued on Aug. 3, 1976 to R. W. Powers, a method of preparing a suspension of additive-free beta-alumina particles is described.
In U.S. Pat. No. 4,166,009 which issued on Aug. 28, 1979 to D. J. Fray, a method for the determination of impurities of specific elements in solid or moltent metal or alloys is described by monitoring the e.m.f. generated between the substance and a reference material. The reference material may be a solid electrolyte comprising beta-alumina containing an element or a solid compound of the element to be detected. A beta-alumina pellet for the probe is formed in situ in one end of an alpha-alumina tube by a hot pressing technique. Sodium aluminate (NaAl.sub.2 O.sub.3) and alpha-Al.sub.2 O.sub.3 powder are well mixed and heated together in air at 1,400.degree. C. after which the mixture is ground to a powder. A carbon rod with a diameter of the internal diameter of the tube is used to cold press the powder at 25 Kg/cm.sub.2 and the load is maintained while the powder is heated to a temperature of 1,150.degree. C. The load and temperature are subsequently increased. Most of the carbon rod is then drilled out of the alpha-Al.sub.2 O.sub.3 tube and the remainder is burnt out using a small oxygen lance, the high temperatures reached during this burning operation help to harden the pellet.
In U.S. Pat. No. 4,499,054 which issued Feb. 12, 1985 to M. Katsura et al, a halogenated hydrocarbon gas detecting element is described comprising a cation source consisting of essentially of beta-alumina, a heater and an ion collector electrode. In the presence of a halogenated gas, the emission of Na.sup.+ ions is increased due to surface interactions. The Na.sup.+ ions are then attracted to the collector electrode by a voltage. In Katsura et al, an increase in the emission of Na.sup.+ ions is observed at times halogenated hydrocarbons and present near the surface of the beta-alumina.